1. Field of the Invention
The present invention relates to a bearing unit for rotatably bearing a rotating shaft or for bearing a rotating body rotatably relative to a shaft, and to a driving motor using the bearing unit.
2. Description of Related Art
As a bearing unit for rotatably bearing a rotating shaft, there has been known a bearing unit configured as shown in FIG. 12. This bearing unit 100, for rotatably bearing a rotating shaft 101, has a structure in which a radial bearing 103 for rotatably bearing the rotating shaft 101 is attached to a metallic housing 102 having a tubular shape opened at its one end. A thrust bearing 104 for bearing in the thrust direction of the rotating shaft 101 rotatably borne by the radial bearing 103 is attached to one open end side of the housing 102.
In the bearing unit 100, a dynamic pressure fluid bearing is used as the radial bearing 103. The dynamic pressure fluid bearing has a structure in which the inner peripheral surface, facing the rotating shaft 101, of the radial bearing 103 is provided with dynamic pressure generating grooves for generating a dynamic pressure.
The housing 102 is filled with a lubricating oil, which is a viscous fluid for generating a dynamic pressure by flowing in the dynamic pressure generating grooves when the rotating shaft 101 is rotated.
The rotating shaft 101 is inserted in the radial bearing 101, and is rotatably borne in the housing 102 with its one end side borne by the thrust bearing 104.
On the other open end side of the housing 102, there is provided a joint part 107 to which a metallic ring-shaped oil seal 105 for preventing the lubricating oil filling the housing 102 from leaking out of the housing 102 is attached. The rotating shaft 101 is projected to the outside of the housing 102 through a shaft passing hole 106 provided in a central portion of the oil seal 105.
A surface active agent is applied to the inner peripheral surface of the oil seal 105 so as to prevent the lubricating oil from being moved through the shaft passing hole 106 to the outside of the housing 102 by a centrifugal force generated upon rotation of the rotating shaft 101 or by other cause.
In the bearing unit 100 configured as shown in FIG. 12, the possible route of outflow of the lubricating oil filling the inside of the housing 102 is only a gap 108 formed between the outer peripheral surface of the rotating shaft 101 and the inner peripheral surface of the shaft passing hole 106 provided in the oil seal 105. Here, by setting the width of the gap 108 to be small, it is ensured that the leakage of the lubricating oil to the exterior of the housing 102 can be prevented by the surface tension of the lubricating oil fronting on the gap 108.
Furthermore, the outer peripheral surface, facing the inner peripheral surface of the shaft passing hole 106, of the rotating shaft 101 is provided with a taper part 109 gradually reduced in diameter along the direction toward the outside of the housing 102. With such a taper part 109 provided, a pressure gradient is created in the gap 108 formed between the outer peripheral surface of the rotating shaft 101 and the inner peripheral surface of the shaft passing hole 106, and due to a centrifugal force generated upon rotation of the rotating shaft 101, a force tending to draw the lubricating oil filling the housing 102 into the inside of the housing 102 is generated. Since the lubricating oil tends to be drawn into the inside of the housing 102 when the rotating shaft 101 is rotated, the lubricating oil penetrates into the dynamic pressure generating grooves of the radial bearing 103 configured as a dynamic pressure fluid bearing, to produce a dynamic pressure, whereby stable bearing of the rotating shaft 101 is realized, and the lubricating oil filling the inside of the housing 102 can be prevented from leaking.
However, the above-mentioned bearing unit 100 has the following problem. When the rotating shaft 101 and the housing 102 are put into relative rotation and a dynamic pressure is thereby generated, the static pressure inside the bearing unit 100 is lowered extremely. The lowering of the pressure inside the housing 102 leads to expansion of residual air, such as a tiny amount of air remaining in the inside of the housing 102 and air dissolved in the viscous fluid, e.g., lubricating oil. Due to the expansion of the air inside the housing 102 or the air dissolved in the viscous fluid, the viscous fluid is pushed out to the exposed side of the rotating shaft 101, possibly leaking to the outside. Thus, in this type of bearing unit 100, the lubricating oil would leak or be difficult to retain, whereby it is made very difficult to achieve favorable lubrication.
Thus, the bearing unit 100 shown in FIG. 12 has the problem that the generation of the dynamic pressure upon relative rotation of the rotating shaft 101 and the housing 102 may cause expansion of the residual air inside the housing 102, leading to leakage of the lubricating oil.
For solving this problem, it may be contemplated to adopt a bearing unit which, in addition to the above-mentioned configuration of bearing unit, is provided with a communicating passage for circulation of air by providing communication between one end side and the other end side in the thrust direction of the rotating shaft projecting from the radial bearing. Such a bearing unit, with the communicating passage, can prevent the pressure on the shaft non-open side from being lowered and, by discharging the residual air inside the housing to the exterior through the shaft passing hole, can prevent the leakage of the lubricating oil.
In addition, the communicating passage can solve the problem of the related art in that it is difficult to insert the shaft after assemblage of the housing and the like, since the clearance between the rotating shaft and the radial bearing is usually as small as a few micrometers and the lubricating oil is already present. To be more specific, the communicating passage plays the role of air vent at the time of insertion of the rotating shaft, thereby facilitating the insertion of the rotating shaft.
However, the communicating passage in the bearing unit as above has been set substantially arcuate in cross-sectional shape, taking into account the ease of manufacturing process. Due to such a sectional shape, there has been the problem that the lubricating oil placed to fill the inside of the housing may stagnate in the air path (communicating hole) to plug up the air path, whereon the communicating hole cannot satisfactorily function as air path.
For more information, refer to Japanese Patent Laid-open No. 2004-132479.